1. Field of the Invention
The present invention relates generally to a method and apparatus for manufacturing non-ceramic insulators of the type having a plurality of trunks and skirts made from a rubbery material and formed around a fiber reinforced plastic core. More specifically, the present invention relates to a method and apparatus for manufacturing elongated non-ceramic insulator used in high voltage power lines.
2. Description of the Related Art
Non-ceramic insulators are often formed with a housing made from elastic insulating material such as silicone rubber molded around a fiber reinforced plastic rod. The rod is reinforced using a plurality of axially aligned fiber bundles which are placed around the plastic rod and treated with a penetrating synthetic resin. This construction produces a rod having great durability and tensile stress. The rubbery elastic insulating material such as silicone rubber, ethylene propylene rubber or the like is used to provide particularly good weather resistance. Non-ceramic insulators constructed in this fashion are particularly long lasting, light weight, and have a high mechanical strength.
Conventional methods of manufacturing non-ceramic insulators include placing the rod shaped core into a cavity of separable molds, and having a rubbery elastic insulating material injected into the cavity to form an insulator housing having a plurality of trunks and skirts which surrounds the insulator core. Japanese Unexamined Patent Publication No. 63-128916 discloses a typical apparatus for manufacturing a non-ceramic insulator.
FIG. 7 shows an apparatus for manufacturing a non-ceramic insulator utilizing a transfer mold method. An upper separable mold 52 is supported on four legs 51. A lower separable mold 53 is supported in the middle portion of legs 51 and is movable along the legs 51. The upper and lower mold 52, 53 include cavities 54, respectively. A molded rubber article or molding is formed by injecting the rubbery material into the cavities 54 of the tightly assembled upper and lower molds 52 and 53.
More particularly, as shown in FIG. 8, a space 56 for receiving a rubbery elastic material prior to the molding process is defined by a piston 57 and a wall in the rear portion of the lower mold 53. The cavities 54 are formed in the front portion of the upper and lower mold 52,53. Flange portions 55, provided at both sides of cavities 54, communicate with the space 56, via corresponding gates 58, respectively. During the molding process, a rubbery elastic material is first placed in the space 56. The lower mold 53 is then lifted by means of a hydraulic lift so as to form a tight fit against the upper mold 52. Heat is next used to soften the rubbery material which is injected into the cavities 54 through the gates 58 by means of piston 57 to form the non-ceramic insulator.
However, according to this transfer mold method, the effective length of the insulator housing is limited to approximately 2 meters. This is due, in part, to the size of manufacturing machines, mold sizing precision, and deflection limits of insulator cores. Nonetheless, some applications, such as ultra high voltage transmission lines, require insulator housings having lengths of over 5 meters. Individual transfer molded insulator housings therefore must be connected in series, by way of flanges, in order to form a sufficiently insulator suitable for use in applications such as with power lines designed for ultra high voltage transmission.
Unfortunately, conventional methods for serially coupling insulator housings together necessitate the use of costly insulator housing alignment techniques which are difficult to properly maintain, and which, if not meticulously maintained, tend to produce misaligned insulator housings. Moreover, since insulator coupling requires the use of flanges or the like for connecting the individual insulator housings, the insulative property of the connected housings is less than that of a unitary housing.